The design and development of an integrated circuit (IC) oftentimes involves extensive testing to ensure that the IC functions correctly. It is common for an IC to include many millions of individual CMOS transistors in various logical arrangements to perform the functions of the IC. The physical size of CMOS transistors is continually shrinking, and gate length as small as 0.13 microns is becoming common. Testing such small discrete elements of an IC is difficult or impossible to perform by physically probing the IC. Moreover, physically probing the IC can easily damage it.
Various technologies exist to test discrete transistors in an IC without physically probing them. One such technology detects faint emissions of light from functioning CMOS transistors. This technology is described in U.S. Pat. No. 5,940,545 (hereafter “the '545 patent”) entitled “Noninvasive Optical Method for Measuring Internal Switching and Other Dynamic Properties of CMOS Circuits,” which is hereby incorporated by reference in its entirety as though fully set forth herein. In some instances, when current flows through a transistor while it is switching, it may emit a photon. Figure A (Background) is a diagram of a CMOS transistor 10 emitting photons 12. The '545 patent describes a technology that can detect and record the location and time of photon emissions from a switching CMOS transistor. A commercially available probe system that employs aspects of the technology described in the '545 patent is the NPTest or Schlumberger IDS PICA (Picosecond Imaging Circuit Analysis) probe system.
Figure B (Background) is a diagram illustrating an example of a photon emission image from the IDS PICA probe system. The image of photon emission data is shown overlaid on a laser scanning microscope (LSM) image of the IC for which the photon emission data was collected. The portion of the IC shown in the LSM image is a four-line inverter block 14 comprising 20 CMOS transistor pairs. A CMOS inverter comprises a complementary pair of an NMOS (or n-channel) transistor and a PMOS (or p-channel) transistor. The dark generally vertical lines correspond with CMOS transistor pairs 16 in the inverter chain. Particularly, one portion of the top first line of the inverter chain comprises a first CMOS transistor pair 18 with a first p-channel region 20 arranged above a first n-channel region 22, and one portion of the second line, below the first line, comprises a second CMOS transistor pair 24 with a second n-channel region 26 arranged above a second p-channel region 28. The n-channel regions of the inverters tend to emit more photons than the p-channel regions. The bright areas 30 surrounded by dark rings are clusters of photon emissions on the image of the photon emission data. A high concentration of photon emissions 32 appears adjacent the n-channel regions (22, 26) of the first and second CMOS transistor pairs (16, 24). Thus, a person viewing the photon emission data overlaid on the LSM image might assume that the high concentration of photon emissions adjacent the transistors were emitted by the two transistors.
With current probe systems, several factors make the identification of photons emitted from a transistor a timely endeavor. Some probe systems employing the '545 patent technology include a time and position resolved photon counting multiplier tube (PMT) to detect single photon emissions from a transistor. With currently available PMT detectors, the probability of detecting a near infrared photon for each switching event is in the range of 10−7 to 10−11 photons per switching event per μm of gate width. The quantum efficiency of the available PMT detectors is poor in the near infrared spectrum, but is higher in the visible spectrum. Processing an IC to collect photon emissions involves removing some, but not all, of the silicon 34 (see Fig. A) over the transistor. The remaining silicon allows transmission of some near infrared spectrum, but blocks the visible spectrum. Thus, the transistors in an IC must perform millions of switches before it is likely that even one photon from each of the transistors is detected.
To exacerbate the very low probability of detecting a photon from a transistor, probe systems also detect background noise photons coming from the probe system itself and from other sources. Thus, transistor photon emissions are mixed with background photon emissions. In many instances, probe systems require the detection of 10 million photons or more (both from transistors and background) before a user can discern whether photons may be attributed to transistors or background. The detection of 10 million or more photons may take hours or days, which in some instances may be prohibitively long.
The photon emission data collected by a probe system may be used to determine the timing characteristics of transistors. In a normally operating CMOS transistor, photon emission is synchronous with current flowing in the channel in the presence of high electric fields. Stated another way, photons are only emitted from a CMOS transistor when it is switching. Thus, the emission of photons from a transistor can be used to extract timing information about the transistor.
To extract timing information for a transistor, the probe system may be used to generate a histogram of the time when photon emissions were detected. One drawback of conventional probe systems is that they lack the ability to process the photon emission data to automatically identify photons that were emitted by transistors. Thus, to obtain a histogram for any particular transistor, conventional probe systems provide a graphical user interface (GUI) for a user to manually define a channel 36 around a portion of the displayed photon emission data that he or she believes may have been emitted by a transistor. The channel 36 is shown as a rectangle in the photon emission image illustrated in Fig. B. To properly locate the channel, typically, the user will compare the photon emission data with a schematic diagram for the IC being tested and define a channel around the photon emissions he or she suspects were emitted by the transistor. The probe system may then generate a histogram for the photons within the channel.
Figure C (Background) illustrates a histogram of the timing pattern for the photons within the channel illustrated in Fig. B. The histogram shows ten photon emission peaks 38 every 10 nanoseconds or so. Each photon emission peaks comprises between about 160 and 200 detected photons at the various time intervals. The histogram also shows numerous other photon emission detections. Because photons emitted from transistors occur at regular intervals and in generally the same location, when enough photon emissions are detected (e.g., 10 million or more) a pattern of photon emission peaks (photon emissions that occurred at about the same time in the same area) may emerge over the background noise for a well-defined channel. Thirty-six million photons were collected to generate the image illustrated in Fig. B and the histogram illustrated in Fig. C. As the background emissions are random, the user may assume that the photon emissions detected at a regular interval are from one or more switching transistors. For testing of the IC, the timing pattern of the photon emission peaks may be used to determine the switching frequency of the transistor, the time when the transistor switched, and may be compared to other transistor photon emission histograms.
Thus, while conventional probe systems provide extremely useful testing information, the time required to obtain that information can be very long.